This invention relates to an alternating current limiting apparatus, and in particular to an apparatus for limiting a transient fault overcurrent which occurs upon the occurrence of a fault such as a ground fault or a line shortcircuit within AC power distribution lines or AC power transmission lines within a power system or between different power systems.
Such an alternating current limiting apparatus is previously known in the art as illustrated in Japanese Pat. No. 987,109 which corresponds to Japanese Patent Application Laid-open No. 50-78850 published on June 26, 1975, and also corresponds with the U.S. patent application Ser. No. 411,888 by John Rosa et al.
This Japanese Pat. No. 987,109 particularly discloses, as shown in FIG. 9 thereof, a bridged alternating current limiting apparatus which is inserted in each phase between two different power systems and is composed of two sets of reactors and capacitors, each set being a series combination of a reactor and a capacitor. The respective interconnections in each series combination are connected through the series combination of a closing device, which is composed of a reverse parallel combination of SCR's, and a resistor.
This conventional alternating current limiting apparatus normally operates as two sets of series resonance circuits, for the line frequency of the different power systems, connected in parallel with each other without the operations of the closing device and the resistor. As a result, the alternating current limiting apparatus serves to directly interconnect the different power systems thereacross. In the case of a fault, the closing device detects an abrupt increase of voltage across the interconnections of the respective series combinations to bridge these interconnections. As a result, two parallel resonance circuits are formed to limit an overcurrent through the different power systems by a large impedance according to the parallel resonances.
Such a conventional series-parallel resonance type apparatus as disclosed in Japanese Pat. No. 987,109 is advantageous in that the transient stability of the systems is improved and the interruption duty for a circuit breaker is lightened because of the low restriking voltage upon the circuit interruption.
However, it is disadvantageous in the following respects:
(1) Since two sets of series resonance circuits of reactors and capacitors are connected in parallel with each other, the circuit arrangement is rather complicated and expensive.
(2) Although there is no problem if the inductance values of both reactors are identical and the capacitance values of both capacitors are also identical, the presence of component variations, if the variations are considerable, will cause a high circulating current to flow through the alternating current limiting apparatus.
(3) If the closing device comprises a non-linear resistive element, both capacitors remain charged so that a discharging resistor or reactor, etc., will have to be provided.
(4) If the closing device comprises a gap or a closing switch, the current limiting effect becomes better as the resistance value of the resistor connected in series with the closing device becomes smaller. Accordingly, it will take a considerable time until the electrical energy stored in the reactors and the capacitors is dissipated after the power systems have been interrupted by a circuit breaker, and so that the limited current has been interrupted.
(5) The resonance of a line inductance in a power transmission line connecting the different power systems and a capacitance between the line and ground provides a high frequency oscillation current which provides an abrupt surge current at a high frequency flowing through a path consisting of one of the capacitors, the resistor, and the other of the capacitors, thereby barely actuating the circuit breaker.